Minimum-shift keying (MSK) is a form of frequency-shift keying with frequency separation equal to one-half the bit rate. MSK provides spectrally efficient modulation in terms of bits-per-second per hertz (Hz) of bandwidth. Prior-art MSK processors are described in U.S. Pat. No. 4,583,048, issued Apr. 15, 1986 in the name of Gumacos et al., and in U.S. Pat. No. 4,635,279, issued Jan. 6, 1987 in the name of Nossen, both of which are hereby incorporated by reference.
MSK signals are demodulated by concurrent use of in-phase (I) and quadrature (Q) demodulators, each of which includes the multiplier. Sinusoidal and co-sinusoidal reference or demodulating signals are applied to the Q and I multipliers, respectively, at a frequency which is ideally equal to that of the carrier frequency of the MSK signal. Asynchronous demodulation can be accomplished with reference signals which are at the same frequency as the MSK carrier, but which are not necessarily in the same phase. This type of asynchronous demodulation results in an increase in noise by comparison with synchronous demodulation, in which the sinusoidal reference signal is in both frequency and phase synchronism with a carrier component of the modulated MSK signal. Increased noise results in increased error rate, and is therefore undesirable.
The MSK signal to be received may be downconverted to baseband in preparation for demodulation. The downconversion is performed by mixing or multiplying a local oscillator signal with the MSK-modulated carrier. If the local oscillator signal is not phase coherent with the MSK carrier, the baseband signal carrier will include a phase drift or phase error. This phase error results in increased noise in the demodulated signal and increases the bit error rate (BER).